The present invention relates to an antenna used in a cellular phone or the like, and a communication terminal using it.
Recently, communication terminals are widely and rapidly used for mobile communications with cellular phones, pagers, and so on.
FIG. 11 shows an example of a conventional antenna used in a communication terminal for mobile communications. In FIG. 11, the antenna is composed of a helical element 100 made of conductive material. One end of the element opens, and the other end is electrically connected to a metal plug 102 having a screw portion. The helical element 100 is encapsulated in a resin cover 101 to assure a mechanical strength and to prevent the metal material from corrosion.
The metal plug 102 is connected electrically with the screw portion driven into a screw hole 103 provided in a case 107. The screw hole 103 is electrically connected to a feeding land 105, which is disposed on the surface of an internal substrate 108, through a spring 104. The feeding land 105 is electrically connected to an RF circuit 106 through a microstrip line 109, so that a current may be fed into the helical element 100 through them.
In a method of radiating the signal supplied to the antenna efficiently as radio wave, it may be considered to eliminate an element loss in a matching circuit by omitting the matching circuit (not shown) which is necessary for matching between the antenna and RF circuit 106. For this purpose, however, the antenna has to be matched with the RF circuit with the structure of the antenna itself.
In the case of the antenna shown in FIG. 11, structural parameters of the antenna adjustable in the impedance include the element length, winding pitch width, and antenna diameter. The more structural parameters, the wider is the adjustable range of the impedance of the antenna, and therefore, that is beneficial for enhancing the radiation efficiency of the antenna.
Lately, in communication terminals for mobile communications, the system becomes more complex. An antenna operable for two frequency bands is required for the antenna for the terminal. To realize such antenna having an excellent radiation efficiency, the number of elements in the matching circuit must be reduced as much as possible. The antenna hence requires more structural parameters for adjusting the impedance of the antenna.
The invention is devised to solve the problems of the prior art, and it is hence an object thereof to provide an antenna having a high radiation efficiency, a widened adjustable range of the impedance, and a capability of matching with the RF circuit without using the matching circuit. The number of structural parameters increases for adjusting the impedance of the antenna.
To achieve the object, an antenna of the present invention and the communication terminal using it are capable of adjusting the impedance of the antenna itself by an inverted winding direction at an arbitrary position of a radiation element. The element wound spirally around an insulating core increases the number of structural parameters of the antenna and expands the adjustable range of the impedance of the antenna.